Mechanical assays
were performed in order to assess the influence of pilot hole diameter versus
screw's inner diameter on screw pullout resistance in the vertebral fixation
systems applied to the vertebral body. The study was conducted in two stages.
In the first, polyurethane test bodies were used for placing 5 and 6 mm screws,
and, in the second stage, the screws were inserted into the lateral surface
of the lumbar vertebral bodies of pigs. The pilot hole was built with drills
with smaller, similar or larger diameter than screws' inner diameter. Mechanical
pullout assays were performed using a universal test machine for the assessment
of maximum pullout screw resistance. The diameter of the pilot hole versus screw's
inner diameter was shown to influence screw pullout resistance.

Keywords: Spine;
Bone screw; Biomechanics

INTRODUCTION

Vertebral fixation
systems are constituted of different components: anchoring components (screws,
hooks, cerclage wires); longitudinal components (nails, plates); transversal
connectors and accessories (washers and nuts). Anchoring components of fixation
systems can be penetrating (screws) and non-penetrating (hooks and cerclage
wires), and act as an anchorage point of fixation systems to vertebrae, over
which correction and neutralization forces are applied (1-4)

Screws have been
commonly used as anchoring elements of vertebral fixation systems and have been
inserted on pedicles, vertebral body and joint mass. Inserting screws on vertebrae
requires opening a pilot hole, of which dimension compared to external or internal
diameter of the screw is related to insertion torque (5-8). Thus,
making a pilot hole may affect biomechanical properties of screws anchored on
vertebrae and influence biomechanical properties of the whole vertebral fixation
system, with potential repercussion on treatment end outcome.

The objective of
this study was to determine a potential influence of the pilot hole diameter
on pullout resistance of implants used for fixing vertebral spine, focusing
the anterior fixation of the vertebral body.

MATERIALS AND
METHODS

One hundred sixty-eight
polyurethane blocks and 60 vertebrae of 150 day-old, 881.20 kg (in average)
male Landrace pigs' lumbar spines were used in the study. Vertebrae were dissected,
separated and kept in a freezer at a mean temperature of -20ºC until assays
could be performed.

The implants used
in this study were the stainless steel, 5-6 mm-wide USS (Synthes) screws (Figure
1), which have been inserted into polyurethane bodies of evidence and at
the lateral surface of vertebral bodies of swine lumbar vertebrae after preparation
of the pilot hole with steel drills. The perforation depth of the pilot hole
corresponded to screws insertion depth, which was standardized at 30 mm for
all bodies of evidence and screw diameters.

The study was performed
in two steps. In the first step, mechanical assays were performed with 5 and
6-mm screws applied to polyurethane bodies of evidence and providing pilot holes
of different diameters above and below screw's inner diameter. The analysis
of the values obtained from mechanical assays performed on polyurethane bodies
of evidence enabled to select a value for pilot hole diameter above and below
the screw's inner diameter to be used during the second study stage.

In the second stage
of the study, 2.5mm; 3.8mm and 4.5mm- wide drills were used for preparing the
pilot hole on bodies of evidence made of bovine bones, and 3.5mm; 4.8mm and
5.5mm drills for 6mm screws. Thus, perforations were made with a smaller, equal
and bigger pilot hole diameter than the inner diameter of the screw.

Mechanical assays
for implants pullout were performed in an Assay Universal Machine (EMIC®).
The bodies of evidence were fixated and the upper portion of the screw was tractioned
by means of a steel wire, over which force was applied for promoting screw pullout
(Figure 2). The test universal machine was attached to a
microcomputer and the 200 kgf load cell, and, by means of a TESC®
software, the applied force was recorded until the implant was pulled out. Mechanical
assays were performed on 12 polyurethane bodies of evidence and on 10 vertebrae
of swine's lumbar spines for each pilot hole diameter studied, in a total of
228 mechanical assays.

The comparison
of results of assays performed on polyurethane bodies of evidence was made by
means of variance analysis (ANOVA) with the objective of checking the existence
of any difference between the averages of drills for each screw alone for each
force and stiffness variables. After ANOVA variance analysis, if a difference
was confirmed between averages, a multiple comparison test (Tukey) was conducted
to check which of the drills were responsible for that difference. A significance
level of 5% (p<0.05%) was adopted for comparing values obtained to the different
perforation widths of the pilot hole.

The comparison
of pullout assays results for screws inserted into swine vertebrae, and the
comparison between groups were performed by a mixed effects linear model method
(fixed and random effects), establishing a significance level of 5% (p<0.05).

RESULTS

The results will
be presented according to the nature of the body of evidence used in the assay
and to screw diameter. The results of pullout test for the 5-mm screws inserted
into polyurethane bodies of evidence are represented on Table
1 and Figure 3.

The pullout test
for the 5-mm screws inserted into polyurethane bodies of evidence showed the
occurrence of an enhancement of the maximum implant pullout force with a reduced
diameter of the pilot hole as compared to screw's inner diameter. A statistical
difference was found between all perforations performed with a diameter smaller
than screw's inner diameter. In a decreasing order of perforation diameters,
a statistical difference was noted between the 3.5 mm perforation and the perforation
corresponding to the screw's inner diameter (3.8mm). The perforation immediately
below 3.5-mm perforation was the 3.2-mm perforation, and a statistical difference
was found among maximum pullout force values between them. No statistical difference
was found between maximum pullout force values with perforations width of 3.2mm
and 3.0mm, as well as between widths of 3 mm, 2.7mm and 2.5mm. Thus, from 3.2mm,
no statistical difference was noted when comparing immediately decreasing perforation
values.

The results of
the pullout assays for 6-mm screws inserted into polyurethane bodies of evidence
are represented on Table 2 and Figure 4.
An increased implants' maximum pullout force was found with the perforation
of a narrower pilot hole than the screw's inner diameter (4.8 mm). A significant
statistical difference was found between all perforation values compared to
screw's inner diameter (4.8mm) and also between the values compared to each
other for smaller perforations than the screw's inner diameter.

A reduction of
pullout maximum force values was seen with a wider pilot hole compared to screw's
inner diameter. The difference found between perforation values smaller than
screw's inner diameter was significant when compared to the screw's inner diameter
(3.8 mm) to the perforation values between each other (4.0mm and 4.5 mm).

The results of
the pullout assays for 5-mm screws inserted into swine vertebral bodies are
represented on Table 3 and Figure 5.
An increased implants' maximum pullout force was found with the perforation
of a 2.5-mm pilot hole, below screw's inner diameter (3.8 mm), but the difference
was not statistically significant. Maximum pullout force values were lower with
the pilot hole perforation with the 4.5-mm drill, which was wider than screw's
inner diameter, and this difference was statistically significant.

The results of
the pullout assays for 6-mm screws inserted into swine vertebral bodies are
shown on Table 4 and Figure 6. An increased
implants' maximum pullout force was found with the perforation of a 3.5-mm pilot
hole (narrower than the screw's inner diameter), and a reduced maximum pullout
force with the perforation of a 5.5-mm pilot hole (wider than the pilot hole).
The differences between implants' maximum pullout force values were statistically
significant for values below and above the screw's inner diameter.

DISCUSSION

Screws belong to
penetrating pullout-resistant implants category, and it has been one of the
most used implants in vertebral fixation surgeries(4). Screws have
different parts: head, outer diameter, inner diameter, threads and thread steps.
The outer diameter is the widest diameter between both outer edges of screws'
threads, and the inner diameter is the width of the screw body over which threads
are fixed (2,4). In general, screws are classified as cortical-type
or spongy screws, according to their threads and inner diameter. Cortical-type
screws have a narrower thread, shorter distance between thread steps and a narrower
diameter. Spongy-type screws have wider threads, longer distance between thread
steps and narrower inner diameter (2,4).

Screws have different
parts with different mechanical functions: head, body and tip. The outer portion
of the screw body is constituted of the thread and the solid inner portion,
which is called screw core (2,4).

The inner portion
of a screw provides resistance to torsion and flexion moments, and is proportional
to the third power of the inner diameter (R=p D3/32).
The threaded portion of the screw is more closely related to screws pullout
resistance (1,4,5).

Screws pullout
resistance is a complex phenomenon, and is also related to other additional
factors to the screw thread, such as quality and density of the bone tissue
and pilot hole (1,7-9).

The pilot hole
is made for guiding and facilitating the introduction of screws into the vertebra.
Screws inserted into the vertebral body make contact to the spongy bone, except
those crossing the opposite cortical. During screw insertion into vertebral
body, the adjacent spongy bone is compacted, producing a stronger interface
between the implant and the adjacent bone, which results in an increased resistance
to implants pullout (4,10).

In previous assays,
we found this correlation between pilot hole diameter and screw's inner diameter
regarding pullout force. In this assay, we changed the pullout force applied,
the screw insertion site, and the kind of body of evidence, and the results
were consistent to previous findings. A reduced pilot hole diameter compared
to screw's inner diameter increases implants' pullout resistance, while an increased
pilot hole's diameter reduces implants' pullout resistance.

The finding about
the behavior of the maximum force required for pulling out implants as the pilot
hole diameter reduced, showed an increased implant's pullout resistance. However,
from certain values on, this difference was not statistically significant, suggesting
that perhaps from a given limit pilot hole diameter value, the impaction ability
of the spongy bone around a vertebral implant does not depend on the pilot hole
diameter anymore. The impaction of the spongy bone occurring around implants
has not been studied, and, so far, long-term biological and biomechanical consequences
of these microfractures produced by implants insertion are unknown.

A pilot hole increase
caused a reduction to implants' pullout resistance, and a statistical difference
was found in all values below pilot hole diameter and also a statistical difference
between different pilot hole diameter values. This result shows a direct correlation
between reduced implants' pullout resistance and an increased pilot hole diameter
compared to screws' inner diameter. As a greater amount of bone is removed during
perforation, a small amount of bone is impacted around the screw, thus weakening
the interface between implant and the surrounding bone, consequently reducing
implants' pullout resistance.

Implants' pullout
resistance is a complex phenomenon, and depends on several factors (4,5,6,10,11).
In the experimental model used here, we sought to simulate the insertion of
implants into homogenous-matrix bodies of evidence, justifying the use of wooden
and polyurethane bodies of evidence, which have been used in experiments related
to this topic. The use of swine vertebrae is related to the difficulty of obtaining
non-osteoporous human vertebrae having similar characteristics regarding bone
density. We cannot leave unmentioned the current medical-legal difficulties
in obtaining cadaver vertebrae for this kind of study, which has been directed
to using animal vertebrae. Nevertheless, the nature of the bodies of evidence
has not interfered on the study's objectives, because we were interested only
in the study of one of the parameters involved in screws' pullout resistance,
and we've been able to establish and repeat the relationship between studied
parameters in different bodies of evidence.

Screws' pullout
resistance is dependent on many factors, but we have been able to establish
the correlation between pilot hole diameter and screw's inner diameter on implants'
pullout resistance. This fact has a wide clinical application and should be
noticed when inserting screws into vertebral bodies intending to achieve the
highest possible implant performance by being aware of its biomechanical properties
and characteristics.